Microorganisms such as the Gram-positive Streptococci and Gram- negative Fusobacteria are causative or contributory agents in the etiology of dental caries, gingivitis and periodontal disease(s). The pathogenicity of these bacteria resides in part in their capacity to form a variety of organic acids and toxic sulfur-containing derivatives as end-products of carbohydrate and amino acid fermentation. These metabolic products are potentially cytotoxic for epithelial and other tissue cells. The aims of this research program are two-fold: (1) to identify and characterize the enzymes that constitute these energy-yielding pathways, and (2) to isolate genes encoding specific metabolic traits in these pathways for use as selectable markers for plasmid or transposon-mediated gene transfer in Fusobacteria. To this end, we have recently purified and characterized two unique phospho-glucosidases from Fusobacterium mortiferum ATCC 25557. The enzymes in question, maltose 6-phosphate hydrolase and phospho-beta-glucosidase, catalyze the hydrolysis of chemically synthesized chromogenic and fluorogenic alpha- and beta- glucoside 6-phosphates. The genes encoding the two enzymes, malH and pbgA respectively, have now been cloned and sequenced. When incorporated behind suitable promoters in an appropriate vector, these genes will be eminently suitable as selection markers for the genetic manipulation and analysis of virulence factors of pathogenic oral Fusobacteria.